Airlines operating in airports having rough runways have experienced serious in-service overheating issues in landing gear of commercial aircraft. Conventional aircraft landing gear is severely challenged during landings, take-offs, and taxiing on rough runways due to the combination of high sliding speeds and high drag loads on the landing gear bearings. However, it is not always practical or cost-effective to maintain runways to preclude rough runway conditions. Low passenger traffic, restricted budgets, and unexpected weather or other challenges can interfere with planned maintenance and repaving projects. Runways and taxiways degrade over time and construction methods, manufacturing procedures, and the availability of maintenance equipment can vary greatly from region to region.
A coherent and sufficient numerical model for predicting, understanding and studying the thermal behavior of the landing gear shock absorber has recently been developed. The analysis methodology relies on a novel analytical mathematical development implemented in a computer-aided simulation framework. For the last 40 years, Computational Fluid Dynamics and the Finite Element Method have been used for many different purposes beyond the application to in-service issues. Efforts to develop a thermo-tribomechanical model of the landing gear shock absorber have been focused on the lower bearing-piston interface where the findings have typically suggested overheating as a primary cause of failure.
In order to prevent in-service overheating issues, it might seem obvious to re-pave or repair runways in certain regions of the world. However, due to various factors such as low passenger traffic and/or limited budget, many regional airports may not be equipped to implement the maintenance standards of heavily frequented airports in other parts of the world.
Therefore, advances in landing gear design to improve reliability, and in particular improvements in landing gear shock absorber systems, are needed.
Overheating bearings are common in other machinery, such as in rotors. Modified bearing geometries, such as the lemon-bore bearing, have been explored. Such studies consider the thermo-elasto-hydrodynamic (TEHD) performance of the bearings. In a TEHD lubrication regime, the heat generation is mainly influenced by the clearance between the contacting surfaces. Compressible lubricants, such as in gas-lubricated bearings, have also been considered. However, the results for rotating machinery are not applicable to the configuration of a slider bearing in a landing gear shock absorber.
Efforts undertaken to address the issues of excessive heat generation in landing gear sliding bearings may be categorized into three strategies: (i) allow the heat generation, but focus on improving the heat evacuation; (ii) withstand the heat generation by improving the material characteristics; or (iii) reduce the heat generation by reducing the bearing friction coefficient, loads, or sliding speed.
Strategies to allow the heat generation may not lead to a significant improvement as the heat generated at the lower bearing interface is concentrated locally and only slowly leaves the zone around the lower bearing. Although materials with higher thermal conductivity and eventual external cooling could lead to improved heat evacuation, the structural characteristics of the system might be changed and the overall weight of the landing gear might increase.
Strategies to withstand the heat generation are impractical for landing gear lower bearings, but are sometimes applied in applications where high amplitude shock-loads (rapidly varying contact pressure) do not occur. In general, the more heat resistant a material is, the less ductile it is. The reported overheating issues must be solved without compromising the structural integrity of the landing gear.
Strategies that reduce heat generation, however, are promising in the context of landing gear lower bearings. Reducing frictional heat generation requires reducing the bearing friction coefficient. In simple terms, the bearing friction coefficient can be reduced either through improved surface characteristics without fluid film lubrication, or through an improved “lubrication mechanism.” In a landing gear system, the configuration (referred to as the configuration C) of the bearing, which is defined by the materials and the geometry, dominates the design, as the input speeds and loads cannot be changed for a given rough runway.
The most promising strategy is to alter the geometry of the configuration C between the lower bearing and the piston, which can be designed for optimal bearing performance. The practice of optimizing the bearing surface is often referred to as the design of the lubrication mechanism. The lubrication mechanism is critical to the design of high efficiency fluid film bearings and is highly application dependent. In practice, the design of the lubrication mechanism is often neglected, due to increased engineering time, which increases the unit development and production cost. Consequently, the most promising solution strategy is the optimization of the lubrication mechanism using the specifically developed computer-aided framework.
In order to prevent thermal issues, it is important to understand the thermo-tribomechanical behavior of an aircraft landing gear shock absorber, and the transient process of heat generation in a phase-changing grease-lubricated lower (slider) bearing.
A conventional main landing gear 100 (e.g., a multi-wheel single-axle main landing gear) is illustrated in isolation in FIG. 1. The landing gear 100 includes a main shock absorbing strut or shock absorber 110 having a lower end 101 attached to an axle 102 mounting two wheels 93, and configured to be pivotally connected to an airframe (not shown). For example, the shock absorber 110 may have a hybrid pneumatic and hydraulic function, sometimes referred to as an oleo strut (or oleo pneumatic strut). The oleo strut includes a piston 104 operatively coupled to the axle 102 and a cylinder 106 that slidably receives the piston 104 and is operatively coupled to the airframe. The piston 104 and cylinder 106 may be connected with a conventional scissors or torque link assembly 94.
Typically the shock absorber 110 is filled with a compressible gas and an incompressible fluid. For example, the gas may be nitrogen which is relatively inert, and the fluid may be of hydraulic kind. When the wheels 93 engage the ground during landing, the aircraft momentum and weight force the piston 104 to slide upwardly in the cylinder 106 compressing the gas and displacing the lubricant. The gas acts as a spring, elastically absorbing some of the energy of the landing. The piston 104 forces hydraulic fluid through flow restrictions in the shock absorber 110, thereby dissipating energy as work and heat, and reducing the tendency of the aircraft to rebound or bounce during landings.
The landing gear 100 shown in FIG. 1 includes a stay 90 having an upper link 91 that is pivotally attached to a lower link 92. The stay 90 extends during deployment of the landing gear 100, and secures the main fitting of the shock absorber 110 in the deployed position. The upper end of the stay 90 is operatively attached to the airframe, and the lower end of the stay 90 is attached near a lower end of the cylinder 106 of the shock absorber 110 in a Cardan joint 99.
A lock stay 95 is also shown, and includes a first link 96 that is pivotally connected to a second link 97. An opposite end of the first link 96 is pivotally connected near an upper end of the cylinder 106, and an opposite end of the second link 97 is pivotally connected to the upper link 91 of the stay 90, near the connection to the stay lower link 92. A downlink actuator 98 is pivotally connected near an upper end of the stay upper link 91, and pivotally connected to the lock stay second link 97.